เช็คให้หน่อยครับทำไม session ไม่ทำงานครับ ขอบคุณครับ

<?
session_start();
include_once 'include/dbconn.php';
ini_set("error_reporting","E_ALL & ~E_NOTICE");
$idsx = $_SESSION['userid'];

$con = mssql_connect($serverName,$uid,$pwd);
mssql_select_db($db,$con) or die ('Error Connect DB User');
?>
<?
if($idsx == ''){
?>
<center>
<form method='post' action='c_login.php'>
<table>
<tr>
<td>ID : </td>
<td><input type='text' name='username' value=''> </td>
</tr>
<tr>
<td>Password : </td>
<td><input type='password' name='password' value=''> </td>
</tr>
<tr>
<td>&nbsp;</td>
<td><input type='submit' value='Login' name='submit'></td>
</tr>
</table>
</form>
</center>
<?
}else{
?>
<?echo $idsx;?>
<p>
<input type="button" onclick="javascript:window.location.href='logout.php';" value="Logout" class="submit">
<?
}
?>

<?
include_once  'include/dbconn.php'  ;

	$id= $_POST['username'];
	$pw= $_POST['password'];
	
	$conn = mssql_connect($serverName,$uid,$pwd);
	mssql_select_db($db,$conn) or die("Not connect to SQL");
	$usernameP=$id;
	$strSQL = "SELECT * FROM [CheckUser].[dbo].[User] WHERE [login]='$usernameP'";
	$objQuery = mssql_query($strSQL);
	$objResult = mssql_fetch_array($objQuery);
	$chkuseridinpoint_em = $objResult['userid'];
	if(!$objResult)
	{
		//echo $strSQL;
			 echo "<script>alert('ไอดีไม่มีอยู่ในระบบ!');</script>";
			 echo '<meta http-equiv="refresh" content="0;URL=index.php">';
			 exit();
			}

	else{

	$passP=md5($pw);
	$strSQL = "SELECT * FROM [CheckUser].[dbo].[User] WHERE [login]='$usernameP' and [password]='$passP'";
	$objQuery = mssql_query($strSQL);
	$objResult = mssql_fetch_array($objQuery);
		if(!$objResult)
				{
			   echo "<script>alert('พาสเวิร์ดผิดพลาด!');</script>";
			   echo '<meta http-equiv="refresh" content="0;URL=index.php">';
			   exit();
				}
	}

					$_SESSION['userid'] = $id;
					session_write_close();
 				    header("location:index.php");
?>